The invention relates to a method for activating a polyphase electric motor which is operated in a star connection, for starting the motor.
Modern electric motors may nowadays be activated by way of electronic controls in a manner which is largely independent of the mains frequency. However, the converter electronics, even with the application of highly integrated electronic circuits, create costs which are often similar to those of the motor. For this reason, one tends to provide activation means for inexpensively manufacturable motors of small and medium power of up to 7500 W for example, which operate with great effectiveness, i.e. at a high efficiency. Permanent magnet motors which are provided with activation electronics constructed of triacs as power switches, are particularly suitable for this on account of their high efficiency. Such triacs are assigned to the individual motor windings as switches, as having the advantage that they are inexpensively manufacturable, have, however, the disadvantage that with usual activation, it is the case of the switches latching, i.e. which after switching on, only block when the current flowing through the switch becomes zero or changes its direction.
Its is also counted as belonging to the state of the art to activate a three-phase brushless electric motor by way of a direct converter (cycloconvertor), which converts the alternating current of the mains supply such that a wave shape of a higher or lower frequency results. Moreover, the rotational speed may be reduced below the synchronous speed by way of a phase-angle control.
So-called soft-starters are also known, with which a brushless three-phase electric motor is started by way of triacs. Such soft-starters are applied in combination with three-phase motors, at whose windings the mains voltage prevails, which may be reduced by phase-angle control.
It is thus counted as belonging to the state of the art, to activate an electric motor supplied with alternating current by way of triacs, in a manner such that the motor not only be operated in a synchronous manner, but also in a super-synchronous and sub-synchronous manner.
However, phase-angle control, as such, is not sufficient to achieve a starting operation with a high moment or and increased torque for a short time. A method for the control of the firing angle of an electric motor supplied with alternating current in a single phase manner, which ensures an increased moment on starting operation of the motor as well as a smooth running once the motor is run up, and operates by way of triacs which suitably subject the main winding and auxiliary winding to current, is known from EP 1 443 635 A1. This method operates with a less complicated electronic control and inexpensive triacs and also permits the rotational speed of the motor to be controlled in quite wide ranges, and this also at a high efficiency.